Line Power for industrial controller

I have a question about the practices used in industrial wiring. I work with indusrial control systems and often I see industrial panels
wired with the line power wired "straight through" the enclosure. By "straight through" I mean the cord passes through a hole drilled in the enclosure. There is strain relief and a metal ring (perhaps for EMI and for a good seal) where the cord enters the enclosure. The enclosures are usually the water resistant type (NEMA 3 or better) and on the male end of the cord is the plug for going into the 110, 220, 400, or whatever the supply voltage is.
Does anybody know why this practice is used? I've seen it most in the U.S.A. I see that it protects agains accidentally unplugging at the enclosure (there is no bulkhead and connector there, but pulling on the cord will still unplug it at the plug end. Additionally, I see that it could be dangerous if the strain relief failed and the wires were pulled out of the terminals inside the panel (possible short).
Any ideas?
-SDS
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You will notice that most wiring in metal boxes has those bushings on it, even in your house. They prevent the wire from moving, without pinching the conductor. If you didn't, then among the less desirable results would be the insulating being abraded off, and a bit of arcing to the box at that point.
Michael

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But why bushings instead of connectors?
Even good bushings are cheap.
Connectors are expensive.
Good bushings with good strain reliefs on the cable are still much cheaper than connectors, and no available connector is designed to withstand a strong lateral pull on the cable that will just bend a Kellems.
-Mike-
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So there's no ANSI or UL standard that reqiures this practice?
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snipped-for-privacy@weberusa.com (Scott S) wrote in message

UL-508A requires the strain relief bushing, but does not appear to prohibit a connector instead of a bushing.
UL-508A is very specific about what kinds of cable you can use for this application. If you permanently install the cord, you control what kind of cable it is. If you put a connector there, the end user could connect whatever cable they want. For example, I have built panels with bulkhead connectors and heavy duty removeable cords. When I went to look at the installation, I found them being powered by 18 AWG hardware store extension cords 100 feet long strung across the factory floor.
You will break the cable or pull the box out of shape before you pull the cable out of a properly-sized Kellems grip.
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